Well, it could start with the Joker causing mayhem in a village in Naruto's world. Naruto, being the hero, tries to reason with the Joker at first. But when he realizes the Joker is beyond reasoning, he uses his ninjutsu skills. He might start with some basic fire - style jutsu to test the Joker's limits and then progress to more powerful attacks like his Nine - Tails Chakra Mode attacks until the Joker is defeated.
The fanfiction could be written by first establishing the Joker's presence in Naruto's world. Maybe he's brought there by an enemy who wants to disrupt Naruto's world. Naruto would then have to figure out the Joker's motives and weaknesses. He could gather information from his friends and allies. Then, when he faces the Joker, he could combine his taijutsu skills with his chakra - based attacks. For example, he could use his speed and strength in a taijutsu fight and then surprise the Joker with a chakra - infused punch. The story could end with Naruto realizing the importance of protecting his world from all kinds of threats, even those from other universes.
Well, one possibility could be that Naruto's powerful ninjutsu skills give him an edge. He might use his Shadow Clone Jutsu to confuse the Joker and then land a fatal blow with a Rasengan. Another aspect could be Naruto's strong sense of justice which drives him to take out the chaotic Joker in the Justice League universe.
Well, it could be that Naruto, with his ninja skills, sees the Joker causing chaos in his world which has somehow merged with the DC universe. He might use his Rasengan to take down the Joker. Maybe the Joker has a new evil plan involving stealing chakra or something, and Naruto just won't stand for it.
It's a really wild idea. On one hand, Naruto has the power to take on the Joker. His chakra abilities like the Rasengan could potentially be a deadly weapon against the Joker. But on the other hand, the Joker is a master of psychological warfare, which Naruto may not be used to dealing with. In a fanfiction, it would be fascinating to see how the writer would make these two characters interact and how Naruto would ultimately defeat the Joker.
Well, in a fanfiction world, anything can be possible. Maybe Harry Potter uses his magic skills like the Killing Curse. But it's really a strange crossover. The Joker is from a completely different universe with no magic, and Harry Potter is from a world full of magic. It would be a very unique and perhaps disturbing story to see how Harry Potter overcomes the Joker's chaos and madness with his magic and finally kills him.
Naruto's speed and strength would be a huge advantage. He can move faster than the eye can see with his ninja training, which would make it hard for the Joker to escape or set up traps. For example, if the Joker tries to throw one of his acid - filled flower things, Naruto could easily dodge and counter - attack.
Maybe a family member of a police officer killed by the Joker. This civilian has a strong sense of justice and a personal vendetta. They are not bound by the same rules as the police and are more likely to take extreme measures. They have seen the destruction the Joker has caused to their family and will stop at nothing to get revenge.
Another possibility is a complete character distortion in the fanfiction. The writer might decide to create a 'bad Naruto' version where he has lost all his positive traits. However, this is quite far - fetched considering the strong foundation of Naruto's character in the original series. It would be a very different and probably controversial take on the Naruto - Hinata relationship.
In a Naruto kills Thanos fanfiction, Naruto's sealing techniques could play a major role. He could use sealing jutsu to seal away Thanos' powers or even Thanos himself. Naruto has experience in dealing with powerful foes and sealing them away. Also, Naruto has the ability to sense danger and react quickly. Thanos, on the other hand, may underestimate Naruto at first. By the time he realizes Naruto's true strength, it could be too late for him.
In fanfiction, Naruto might kill a god through the power of his strong will and the unique abilities he has. For example, his Sage Mode could be enhanced to a level that allows him to overpower a god - like entity. Also, his Nine - Tails chakra could be combined with some new, made - up powers in the fanfiction world to give him the strength needed.
Well, in a fanfiction scenario, Naruto might kill Aizen if he manages to use his shadow clones to outwit Aizen's illusions. Naruto could also use his Sage Mode to sense Aizen's reiatsu and counter his attacks. Maybe he combines his Rasengan with some new jutsu he's developed specifically to deal with Aizen's powers.